1. Field of the Invention
The present invention relates, in general, to a diffusion lens for diffusing light emitted from a light emitting diode and, more particularly, to a diffusion lens for diffusing light emitted from a light emitting diode, which uniformly diffuses light that is emitted from a light emitting diode, employed as a light source for backlighting a liquid crystal display, in the central direction, the diagonal direction and lateral direction of the lens.
2. Description of the Related Art
A display device, lacking a function of emitting light by itself, such as a Liquid Crystal Display (LCD), must use a separate light source for backlighting. For this function, a fluorescent lamp, such as Cold Cathode Fluorescent Lamp (CCFL), is mainly used in the prior art. However, recently, the brightness and performance of a Light Emitting Diode (hereinafter referred to as an “LED”) have been improved, and instances in which LEDs are applied for backlighting LCDs, have increased.
However, since an LED is a point light source that emits light from a single point, a diffusion lens (also called a “secondary lens”) for diffusing light, emitted from an LED chip, is used to widen the illumination area and obtain uniform luminance in spite of the short distance between the LCD and the light source.
FIG. 1 illustrates the shape of a conventional diffusion lens for diffusing LED light, and the trace of the LED light.
As shown in FIG. 1, the conventional diffusion lens for diffusing LED light includes a bottom surface 10 for receiving an LED chip L, a first lens surface 20 upwardly inclined from the central axis of the lens, a second lens surface 30 extending from the outer end of the first lens surface 20 and downwardly inclined toward the central axis of the lens, and a third lens surface 40 curved from the lower end of the second lens surface 30 to the bottom surface.
First, in the bottom surface 10 of the lens, a receiving cavity 15 for receiving the LED chip L is formed, so that LED light is incident on the lens along the boundary surface of the recess.
The first lens surface 20 forms an inclined surface upwardly extending from the central axis (indicated by the dotted line) of the lens, and forms, together with the second lens surface 30, a funnel-shaped protruding surface. Accordingly, in a range adjacent to the central axis of the lens, LED light, having passed through the bottom surface 10, is incident on the first lens surface 20 at an angle lower than a critical angle of θc, and is refracted in the central direction of the lens. In contrast, when the incidence angle of the LED light is equal to or greater than the critical angle, the LED light is totally reflected from the first lens surface 20, passes through the second lens surface 30, and is then diffused in the lateral direction of the lens.
As shown in FIG. 1, the second lens surface 30 and the third lens surface 40 form a depressed part on the side surface of the lens, and the LED light, directly incident on the second lens surface 30, is refracted from the second lens surface, and advances in the lateral direction of the lens.
Since the conventional diffusion lens has a very complicated shape, it is not practical to produce the diffusion lens by polishing and processing glass, so the diffusion lens is generally produced through injection using a plastic material as the material. However, there is a disadvantage in that, even if a plastic injection process is employed, it is difficult to eject a plastic lens from a mold after the molding of the plastic lens has been completed, because of the funnel-shaped protrusion and the depressed part on the side surface, and it is also not easy to produce the mold of the lens.
Further, since LED light incident on the first lens surface 20 is refracted to the top of the lens in a range adjacent to the central axis of the lens, or is totally reflected when the LED light is incident on a side surface compared to the central axis, it is difficult to adjust the amount of light of illumination passing through the center portion of the lens, using upper refracted light, refracted from the first lens surface. That is, most LED light incident on the first lens surface is totally reflected, and refraction is very limitedly conducted only around the central axis of the lens, so that a phenomenon in which the center portion of the lens is darker than the surrounding portion of the lens may easily occur.